Tree-Ring Dating of Alpine Fault Earthquakes, New Zealand

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9316428 Bull Tree-ring dating of recent New Zealand earthquakes is an important and robust test of lichenometric dating of regional rockfall events caused by earthquakes. Lichen studies indicate that about 240 years have elapsed since the most recent Alpine fault earthquake, and a magnitude 8.1 +0.2 earthquake may soon rupture about 400 km of the Alpine fault. Verification of lichenometry earthquake dates would substantiate needs for establishing seismic monitoring and earthquake hazard reduction programs. Small patches of trees of the same age are typical of the rain forest between the Alpine fault and the seacoast in the South Westland district of New Zealand. Partial destruction of the forest canopy by disturbance events, such as windstorms or earthquakes, allows a new generation of rimu (Dacrydium cupressinum) to germinate and fill the gaps. Regeneration patches appear to be larger on poorly drained (seismically sensitive) soils of the Okarito Forest near the coast where we will make our study. Increment-core estimates of germination times for even-age patches of rimu in two plots at a more inland site were made by Boyd Cornere. His M.Sc. thesis suggests that both plots had a canopy- opening disturbance approximately 250 years ago. These preliminary results encourage us to accurately date forest-disturbance events through study of times of abrupt increase or decrease of tree-ring widths. The proposed dendrochronology data set is a by-product of selective logging in the Okarito sustained-yield forest. An initial phase of our work will be to develop a model for spatial patterns of tree populations ages. This important task will allow us to make predictions about the way seismic shaking will affect tree-ring width. About 1000 mature rimu trees will be harvested in 1993 and 1994. Departures from a standard chronology for rimu will be identified by anomalous patterns of tree-ring widths and densities. We will select, prepare, and analyz e 200-300 sections from basal rimu trunks for dates of canopy-opening events, tree- damage events, and germination dates. Year 1--Stand dynamics study of the Okarito Forest; sample collection, shipping, and preparation; measuring and cross-dating of tree rings. Year 2-- studies and dating of tree-ring anomalies, analysis of data, and report preparation. Our project has multidisciplinary significance. Comparison of tree-ring and lichenometric dates should validate a new surface- exposure dating method that is more accurate than radiocarbon dating. We expect to determine the earthquake recurrence intervals for the adjacent Cook and Brunner segments of the Alpine fault, and the times since the most recent earthquakes. This will test the hypothesis that a major earthquake is imminent. An ecological end product will be improved understanding of forest dynamics and microenvironments, and forest-landforms relations.